Riding the Highways of Light

Riding the Highways of Light

Science mimics science fiction as
a working model flying disc - a "Lightcraft" - takes
to the air

April 16, 1999: It looks like another fine product
of Area 51, and it really is shaped like a flying disc, and would
even fly like one. If it works, the family tree will trace back
not to Roswell, New Mexico, but Troy, New York.

"It came out
of a trans-atmospheric vehicle design course at Rensselaer Polytechnic
Institute," which is in Troy, explained Prof. Leik Myrabo.

The microwave Lightcraft being studied by Myrabo and his students
is shaped that way because that's how the physics works. It's
an advanced derivative of a tiny, 25-gram craft that he is pushing
around on a 10 kilowatt beam of infrared laser light in tests
at White Sands Missile Range, N.M.

Myrabo discussed his work last week during the Advanced Propulsion
Research Workshop held in Huntsville.

"This is where we are now," Myrabo
said, showing a picture of Dr. Robert Goddard with his first
liquid propellant rocket, launched March 16, 1926. Just 43 years
later, a sophisticated descendant of that rocket sent the first
humans to the Moon.

"My goal has been to cut the cost of getting to space
by a factor of 1,000 using a system that is completely green,"
he explained of his passion for the past three decades. Since
1972, he has been building on an idea developed by Arthur Kantrowitz
to use lasers to launch satellites. Myrabo introduced a variation
using the atmosphere as the propellant heated by a laser. At
higher altitude and at 5.5 times the speed of sound, as the air
thins, the craft would use a small supply of on board hydrogen,
still heated by the remote laser beam.

Myrabo's initial design for
NASA was a 5-meter (16.5 ft) diameter, four-person craft - "ma
and pa in the front, two kids and a dog in the back" - in
a shape that he dubbed Acorn. The front is shaped to reflect
the coherent laser light into a narrow region between the body
and a shroud. The focused light superheats the air to become
a jet exhaust that pushes the craft up. In this case, the laser
power station would be based in orbit.

He next worked on the Toy Top design which reversed the
optics for lasers based on the ground. The Strategic Defense
Initiative Organization was interested in this approach for rapid
launches of satellites weighing around 100 kg (220 lbs).

Under continued Air Force and NASA sponsorship, Myrabo has
developed and test flown a 15 cm (6 inch) diameter model of the
Toy Top Lightcraft.

"We just passed Goddard's second flight of 92 feet (28
meters)," Myrabo said. That limit, though, is set at 120
ft (37 m) by a light shield erected by a crane to stop the light
beam and eliminate the chance of blinding a satellite sensor.

The next step is to develop
a 150 kilowatt laser that would boost a larger model to 30 km
(18 mi) altitude. Eventually, a 1 gigawatt laser would be needed
to orbit satellites as Kantrowitz, Myrabo, and others have long
envisioned.

It's possible, though, that it all might be supplanted by
mid-21st century by the microwave Lightcraft. When word of his
work got around to the Space Studies Institute in Princeton,
Myrabo was asked if he could design a similar craft that used
microwaves beamed from space since microwave transmitters were
a more mature technology than lasers.

Myrabo (right) watches as students
at Rensselaer run a computer simulation for the air spike wind
tunnel test on the Lightcraft model held by the student at left.
(RPI photo)

The concept that evolved is a part airship, microwave receiver,
and (the smallest part) jet and rocket engine, and as green as
any space concept. The 12-person, 20-meter (66 ft) craft would
be powered from the Earth's surface to the Moon by sunlight captured
by an orbiting power station (1 km diameter, 20 GW power), converted
to microwaves, and beamed to rectennas (rectifying antennas)
that turn it back into electricity on the Lightcraft. That's
where the saucer shape comes from.

Other Propulsion
Stories this week

Apr 6: Ion Propulsion -- 50 Years in the Making
-
The concept of ion propulsion,
currently being demonstrated on the Deep Space 1 mission, goes
back to the very beginning of NASA and beyond. April
6:
Far
Out Space Propulsion Conference Blasts Off - Atoms locked in snow, a teaspoon from the heart
of the sun, and the stuff that drives a starship will be on the
agenda of an advanced space propulsion conference that opens
today in Huntsville.April 7: Darwinian
Design - Survival of
the Fittest SpacecraftApril 7: Coach-class
tickets for space? - Scientists
discuss new ideas for high-performance, low-cost space transportationApril 8: Setting
Sail for the Stars - Cracking
the whip and unfurling gray sails are among new techniques under
discussion at the 1999 Advanced Propulsion Research WorkshopApril 12: Reaching
for the stars - Scientists
examine using antimatter and fusion to propel future spacecraft.April 16:
Riding
the Highways of Light - Science
mimics science fiction as a Rensselaer Professor builds and tests
a working model flying disc. The disc, or "Lightcraft,"
is an early prototype for Earth-friendly spacecraft of the future.Â

The airship part is a pressurized helium balloon-type
structure made of advanced silicon carbide film (transparent
to microwaves) to make the craft partly buoyant and to provide
for a large parabolic reflector for the energy beamed from space.
The craft would be encircled by two superconducting magnet rings
and a series of ion engines, and topped with solar cells.

At launch, the Lightcraft would use electricity from its solar
cells (powered by an infrared space-based laser at night) to
ionize the air and move the craft through electrostatic discharges.
The craft could move at 80 to 160 km/h (50-100 mph).

That's just low gear. Switching on the microwave transmitter
would make the Lightcraft disappear in less than an eye blink.
The microwaves would be focused by the internal reflector to
heat the air on one side or the other of the craft and push it
in the opposite direction.

"This is used to climb out to a good altitude and beyond
the speed of sound where you use the magnetohydrodynamic drive,"
Myrabo continued. Now the craft tilts from flying edgewise to
flying flat into the air stream. That seems wrong but for another
trick. The microwaves are reflected forward to create a superhot
bubble of air above the craft and form an air spike that acts
as the nose cone as the Lightcraft accelerates to 25 times the
speed of sound.

"This cleans up the aerodynamics of a vehicle that does
not look like it should fly in that direction," Myrabo said.
Even better, when the load is properly balanced the craft sails
through the air without leaving a shock wave and virtually no
supersonic wake. Water is used by the craft to cool the rectennas
and as a propellant in the last stages of ascent.

At least initially, during the
prototype phase, it won't be for everyone, just NASA and military
test pilots. The hyper-energetic performance will require that
the crew ride in liquid-filled escape pods to protect them from
g-forces greater than even fighter pilots occasionally endure.
In some Air Force Space Command schemes, the crew would breath
an oxygenated fluid to protect their lungs.

It all sounds a bit too much like
science fiction, but Myrabo points out that most of the technologies
or principles have been demonstrated. Faculty and students at
Rensselaer have demonstrated the MHD slipstream accelerator and
the air spike concept in a high-speed wind tunnel, and will test
new models of other parts of the propulsion system later this
year.

"If successful, this will cut the cost of getting to
space to whatever someone wants to charge for electricity from
the orbiting power station," Myrabo said. "You could
go halfway around the world in 45 minutes, or from the Earth
to the Moon in about 5-1/2 hours."

At the Moon, the Lightcraft would zoom down a series of ring-shaped
electromagnets that would slow the craft, or could accelerate
another Lightcraft for the return to Earth.

"This would require a fully mature infrastructure to
support these vehicles," Myrabo said. But it could bring
about an era of airline-like space travel on "highways of
Light."

More information about the Lightcraft program, including additional
images and Quicktime movies, are available at the Lightcraft
web site at RPI.

Spacecraft
may fly on "empty"
(Jan. 22, 1999) Using a propulsive tether concept, spacecraft
may be able to brake or boost their orbits without using on board
fuel. A NASA/Marshall project, named "ProSEDS," is
slated to demonstrate braking, by accelerating an expended rocket
toward re-entry.